1. Field of the Invention
This invention relates to time scale generation and particularly relates to traceable time scale generation.
2. Background of the Invention
In many applications such as telephone networks, power networks, and computer networks, clock signals are generated for controlling timing. For instance, LOw frequency, RAdio Navigation (Loran) and Global Positioning Satellites (GPS) may be used as clock sources. Although Loran and GPS provide inexpensive and precise time scale sources, there are several drawbacks that have resulted in such sources not generally being used. Typically, Loran and GPS master clocks are considered undesirable because the communication link to the source--a radio signal--provides a relatively noisy communication link. Such noisy communication links result in injecting noise into the master clock as perceived at the node. Typically, rather than using these external timing sources, many of these clock signals are generated at each switch or node in the network. Clock signals generated at a switch or node provide a local time base for tracking and controlling the relative occurrence of events. Examples of such time scale generators include ceramic oscillators, crystal oscillators, quartz oscillators, rubidium oscillators, and cesium oscillators. Cesium oscillators, which are so-called primary atomic clocks, typically provide time base references with the highest degree of precision, accuracy and stability, but at the greatest cost.
Each of these prior art oscillators may be used as a local time base generator and has a precision and stability that may depend on a variety of factors such as temperature, noise, system biases, etc. When it is necessary for many of the nodes of the network to communicate with each other, there typically is a requirement that these individual nodes have to be operating in synchronization or syntonisation with each other. Synchronization means that measured against some reference, there is a specific timing (phase and frequency) relationship with a given level of precision between the clocks at two different nodes. Syntonisation means that measured at a given level of precision the frequency of the clock sources at two different nodes is the same.
Various types of master or reference sources may be used. For example, the master clock source such as a cesium clock may be a cesium oscillator that is distributed over the network to each node. Each node in the network may incorporate a less stable oscillator such as a rubidium oscillator and receives the master clock signal over a communication link.
To lock such oscillators to the master or reference clock source, various high precision phase and frequency locked loops such as the Stratum Model ST2, ST3 and ST3E which are available from the assignee of this application have been developed. These clock circuits provide high precision tracking of the reference or master signal with accuracy in the range of one part in 10.sup.11 over a day. However, depending upon the amount of precision and accuracy desired, the use of these oscillators results in increased complexity and greater cost.
The most common method of obtaining synchronization or syntonisation is to use phase locked loops or frequency locked loops, respectively, that are locked to a reference or master clock signal that is distributed throughout the network. A disadvantage of such distribution of a master clock signal throughout the network is that noise in the communication link and/or various error sources in the phase or frequency locked loop may cause timing (phase) errors or frequency errors in the network. Such timing or frequency errors result in slippage in the timing of data or other types of information frames between various nodes. Where phase or frequency errors occur, this may result in slippage of, for example, timing intervals, resulting in the loss of data.
As a further means of improving local time scale generation, various processes have been developed for improving stability. One common method for improved time scale generation is to have several highly accurate different sources such as oven-based rubidium oscillators and average the signals of the oscillators to try to minimize errors statistically.
A further improvement of such averaging techniques is ensemble time base generation. In ensemble time base generation, a variety of clock and other time base sources are provided. Rather than taking a simple mathematical average of the timing source, various weighing factors are taken into account for generation of the time scale based upon either the predicted or measured accuracy and stability of the various different time sources. There are several examples of ensemble time scale generation methods including public domain methods such as the AT1 and the AT2, which were developed by the National Institute of Standards and Technology (which is part of the United States Department of Commerce) and described in M. Weiss & T. Weissert, "AT2, A New Time Scale Algorithm: AT1 Plus Frequency Variance," Metrologia No. 28 p. 65-74 (1991).
Nonetheless, ensemble time scale generation fails to provide either synchronization or syntonisation. Therefore, it is a first object of this invention to provide a synchronizable or syntonisable time scale generator to provide synchronization or syntonisation throughout a network. It is a further object to provide a time scale generator using a straightforward and relatively inexpensive architecture. It is a further object to provide a disciplined time scale generator that disciplines the ensemble time source to a reference clock. It is yet a further object of the invention to use GPS timing to provide the reference or master clock signal.